Our studies have been aimed at determining the molecular mechanism by which reticulum cell sarcoma (RCS) stimulate syngeneic lymphocytes from SJL/J mice. Our preliminary studies have shown that antigens coded for by the I region of the MHC are expressed on RCS tumor cells but not on host cells. These antigens are recognized by the antitumor lymphocytes and also are recognized by certain anti-allogeneic lymphocytes. We propose to investigate the role of the tumor antigens in the induction of the antitumor response and in the regulation of the host antitumor response. Our working hypothesis is that there exists a cooperation between tumor cells and host lymphocytes which is important for regulation of tumor growth. The syngeneic response generates several lymphokines which in turn regulate tumor growth. Our studies are aimed at characterizing the nature of the various cell-cell interactions and role of lymphokines in tumor development.